In the USDA:APHIS:VS:NAHMS Dairy 2007 Study, 10.8% of operations were positive for Salmonella in bulk-tank milk samples tested by RT-PCR. In contrast, when in-line milk filters were included for testing in the 2007 study in addition to bulk-tank-milk sample, 28.1% of operations were positive for Salmonella.
In 2007, 8.8% of operations tested positive for Listeria species when a single bulk-tank-milk sample was cultured. With the addition of in-line milk filter testing in 2007, 32.1% of operations tested positive for Listeria species.
Testing of in-line milk filters in addition to bulk tank milk increases the sensitivity of detecting Salmonella and Listeria species. In-line milk filters entrap and concentrate pathogens from gallons of milk in one sample, which makes them a more sensitive and suitable sample for screening pathogens compared to bulk tank milk alone.
Although the widespread distribution of Salmonella and Listeria monocytogenes in the dairy environment hampers the control of both bacteria, there are factors associated with their presence in bulk tank milk that dairy farmers should monitor in order to eliminate these bacteria from milking systems and decrease Salmonella prevalence on dairy operations.
- Implementing recommended milking hygiene practices, such as ensuring that teats are clean and using a teat disinfectant prior to milking, should decrease contamination of milk with these pathogens.
- Testing new replacement heifers before they are incorporated into the herd.
- Proper sanitation of maternity and calf rearing areas.
- Control of birds and rodents.
Practices that help decrease the prevalence of Listeria monocytogenes include:
- Feeding cattle good quality silage.
- Preventing contact with manure from infected animals.
- Thorough cleaning and disinfection of bulk tanks.